The Seebeck effect, or the thermoelectric effect, is the voltage difference that exists between two points of a material when a temperature gradient is established between those points. Materials, usually semiconductors or conductors which exhibit this phenomenon, are known as thermoelectrics or thermoelectric materials. Devices made from thermoelectric materials take advantage of the Seebeck effect to convert heat into electricity. For instance, the Seebeck effect is the physical basis for a thermocouple, which is often used in temperature measurement.
Measurements of the Seebeck effect are reported as the Seebeck coefficient (α) in units of μV/K (microvolts per Kelvin). The Seebeck coefficient can be defined as the ratio between the open circuit voltage and the temperature difference between two points on a conductor, when a temperature difference exists between those points. The Seebeck coefficient can take either positive or negative values depending upon whether the charge carriers are holes or electrons. The Seebeck coefficient is often referred to as the thermoelectric power or thermopower.
Good thermoelectric materials should possess Seebeck coefficients with large absolute values, high electrical conductivity (σ, in units of Ω cm), and low thermal conductivity (λ, in units of W/cm K). A high electrical conductivity results in minimizing Joule heating in the thermoelectric material, while a low thermal conductivity helps to maintain large temperature gradients in the material.
The efficiency of a thermoelectric material is, therefore, described by the thermoelectric figure-of-merit (Z, in units of K−1), which is calculated by the relationship:
  Z  =                    α                  i          σ                    λ        .  A useful dimensionless figure-of-merit is defined as ZT, where T is temperature (in K), and
  ZT  =                    α                  2                      σ            ⁢                                                  ⁢            T                              λ        .  
Metals and metal alloys received much interest in the early development of thermoelectric applications, but these materials have a high thermal conductivity. Furthermore, the Seebeck coefficient of most metals is on the order of 10 μV/K, or less. Semiconductors were found with Seebeck coefficients greater than 100 μV/K. Generally, semiconductors also possess high electrical conductivity and low thermal conductivity, which further increases Z, and thus increases the efficiency of the thermoelectric material.
For instance, bismuth telluride (Bi2Te3) and lead telluride (PbTe) are two commonly used semiconductor thermoelectric materials with optimized Seebeck coefficients greater than 200 μV/K.
Optimizing the Seebeck coefficient of a material generally involves synthetic methods by which the stoichiometry of the starting material is slightly altered with a dopant material. Often, this leads to a material with an entirely different composition. In addition, there is no easy way to predict the Seebeck coefficient of a specific material composition.
Accordingly, there remains a need for materials with Seebeck coefficients with large absolute values. In addition, there remains a need for a method to increase the Seebeck coefficient of a material that does not necessarily involve adding dopants to the material. Embodiments herein address these and other needs.